1. Field of the Invention
This invention relates to an endodontic instrument and, more particularly, to an instrument for treating a root canal such as a reamer, a file, or the like.
2. Description of Related Art
A root canal in a tooth is very narrow and has a delicately curved shape, which is individually deviated greatly. A reamer and a file are generally used as an endodontic instrument for grinding and shaping such a root canal. The reamer and the file are cutting devices having blades extending in a spiral shape, in which contour connecting the apex extending in a tapered shape. The reamer is mainly controlled to rotate to grind the root canals while the file is mainly controlled to be pushed to grind the root canals. Among such files, there are K files having relatively small twisting angles but capable of grinding by rotation and H files having the largest twisting angle used exclusively for grinding by pushing.
When a root canal therapy is made in use of, e.g., a file, a medical doctor grips the file by his fingers to grind and to shape the root canal by application of pushing and pulling control or slight rotating control. The doctor does therapeutic treatments without watching the area of the root canal ground by the file. Therefore, such treatments require skills and create various high level demands to performance and characteristics of files.
For example, as required characteristics for file, exemplified are to be able to flexibly follow curved shapes of root canals which are individually deviated greatly, to have good grinding property, to be able to easily remove the ground chips according to pushing and pulling control or rotating control, to have proper resistance responding to the canal shapes and instrument sizes, to have high breakdown angle property against torsional force, and so on. Some files have been proposed to satisfy such demands.
FIG. 5 is a diagram showing cross sections of some files commercially available. A circle shown in FIG. 5 is a circle with which the blades of the file at arbitrary positions are in contact. In the drawings, numeral 51 indicates a file having a square cross section and having a high cross-sectional secondary moment among the files commercially available. Therefore, the file provides high resistance against twisting and bending, but has a smaller rake angle (angle between the blade 51a and the circle), bad property of cutting and removal of cut chips, a lower traceability to a root canal.
Numeral 52 is a file having a triangle cross section and has a low cross-sectional secondary moment and a good traceability to a root canal. The blade 52a of the file has a large rake angle, and the file can have a large space within the circle, so that the file can have good property of cutting and removal of cut chips. However, the resistance against bending and twisting of a file is too low to transfer the force of the dentist to the blade as to cause a problem on the cutting capability, and also the blade may have slightly inferior durability. Numeral 53 is a file having a rhomb cross section, which has good property of cutting and removal of cut chips and has a proper resistance against bending and twisting because the file has a wider rake angle of the blade 53a than that of the file 51 having a square cross section to create a large space within the circle. Accordingly, the file can be used preferably for grinding the root canals.
The file in the rhomb cross section is not free from suffering some problem, and the durability as a blade in cutting work and the balance in proper flexibility and, conversely, proper rigidity and high cutting capability are still unsatisfactory. Moreover, forming such files into an accurate rhomb shape brings high costs to render the products expensive.